There are substantially two modern method for carrying out leak detection of a fuel assembly for nuclear light-water reactors. INMAST-sipping for PWR-reactor plants and TELESCOPE-sipping for BWR-reactor plants (BWR=Boiling-Water Reactor).
A leakage occurring leads to the reactor water and hence the different parts of the primary circuit of the reactor being contaminated with the radioactive fission products. When a contamination of the reactor water has been determined or is suspected to have occurred, it is of the utmost importance that the leakage is locted so that leaky fuel assemblys can be replaced and later repaired. A fuel assembly includes most often several fuel rods.
The so called INMAST-sipping is used for PWR-reactor plants and is a sipping method of conventional type, where a so called on-line gas detection is used.
At the INMAST-sipping, the fuel assembly suspected to be leaking is drawn into a suction hood, which preferably is the refuelling machine mast. The fission gases emitted from the fuel assembly are sampled in the upper part of the hood, and thereafter detected in a gas detection circuit. The gas volume in which an amount of fission gas from a damaged fuel assembly is emitted is quite voluminous. This gives the result that the detection sensitivity is quite low.
The FR-2509898 describes a fuel leak detection for a PWR in which the unit of a fuel assembly drawn into a suction hood is lifted up several meters well above the reactor core in order to increase the internal relative pressure of the rods in relation to the water pressure but so that the fuel assembly is still surrounded by water. The gas emitted in the water and then in a gas volume above the water level in the hood is sucked from the upper part of the hood, which then is closed but for the gas sucking device. The possible content of radiactive fission products in the sucked gas is examined. The detection sensitivity is quite low. In order to increase the same a gas stream is forced through the water around the fuel assembly in the hood from its bottom to the top. However, the detection sensitivity is quite low even with this measure.
At the so called TELESCOPE-sipping, which is totally adapted to a BWR-reactor plant, a great amount of the water surrounding a damaged fuel assembly is pumped from a nozzle placed at or a doom placed around the upper part of the fuel assembly to be examined lifted up somewhat from the reactor core by a gripper at a telescope mast arrangement from the reactor core and water pumped from the nozzle is supplied to a measuring circuit having a little volume. The water is degased in order to make a gas detection on-line. The detection has a high measuring sensitivity.
This kind of sipping is described in the Swedish Patent No. 91015065, according to which a hood or a nozzle arrangement is placed in the region around the gripper at the upper part of a lifted fuel assembly. A pump sucks water from that region. The fuel assembly of a BWR-reactor plant is in itself closed to the surroundings so that the pumped water from the nozzle to a great extent comes from water streaming through the inside of the assembly. The fuel assembly lifted-up is through-flushed with reactor water and the gripper is rinsed-off. This is also the case when the fuel assembly, after having been lifted to a given position vertically, is held in this position or is relowered, and the analysis of leaking fission gases is then carried out.
The fuel assemblies in a PWR-reactor plant have quite another open structure than the closed assemblies in a BWR-reactor plant. A nozzle at the upper part of a fuel assembly of a PWR could not possibly have the function to collect water streaming around the fuel rods in the fuel assembly. The radiation is emitted from its sides. Therefore, the fuel assemblies of a PWR-reactor plant must be placed in a closed space when lifted from the reactor core.
An object of the invention is to provide a fuel leak detection for a PWR with a high sensitivity.
According to the invention the gas measuring circuit at an INMAST-sipping device is replaced by a water sampling circuit. Water surrounding the fuel assembly to be examined and then placed inside a lifting rod is pumped from the water inside of the rod to a water/gas separation device. The separated gas is then detected on-line in the same way as for the so called TELESCOPE-sipping for BWR-reactor plants.